The invention relates to a process for producing seamless tubes of austenitic steels containing chromium and nickel.
Austenitic tubes may be made by a crossrolling process, by extrusion, by continuous casting or by welding of sheet-metal strips. While it is true that welded tubes are less costly to manufacture, because of the welding seam which results they have a relatively small range of applications. In the crossrolling process a mandrel must be driven through a corresponding bloom, and efficient application is obtained only in certain cross sections.
In the manufacture of seamless tubes by extrusion, a heated steel is forced through a die, where very intense deformations occur. For austenitic tube quality, it has been necessary for the steel to undergo a corresponding deformation before being subjected to extrusion, in order to obtain a corresponding change in structure and an improvement in ductility.
It is known that steels having a high chromium and nickel content may be processed by continuous casting into seamless tubes of any desired length. An overheated mass of molten steel is fed continuously into a mold which is rotating about its axis. The molten steel is pressed against the mold by centrifugal force and, when delivery of the metallic melt is appropriately limited, an axially symmetrical cavity may be obtained. However, this process permits only tubes having a relatively great diameter to be produced, and the mechanical and metallurgical properties of the resulting tubes are not satisfactory.